1. Field of Invention
The present invention relates to a mask-printing method and a mask-printing apparatus and particularly to the art of dealing with a defective printing.
2. Related Art Statement
A mask printing is carried out for, e.g., printing a creamed solder on a printed wiring board (xe2x80x9cPWBxe2x80x9d). More specifically described, a mask having one or more through-holes is superposed on the PWB, and the creamed solder is moved on the mask and is squeezed into the through-holes of the mask, so that the creamed solder is applied to the PWB. However, a defective mask printing may occur. A defective mask printing may be a printing with an insufficient amount of creamed solder, or a printing at an incorrect position on the PWB. To deal with this problem, Japanese Patent Application laid open under Publication No. 5-50578 teaches inspecting whether a creamed solder has been appropriately printed on a PWB and, if a defective printing is found, solving the cause of the defective printing to prevent the same defective printing from repeating two or more times.
Thus, the prior-art document teaches solving the cause of the defective printing. However, the PWB with the defective printing is dealt with as a defective object, and is discarded out of a production line.
The present invention provides a mask-printing method and a mask-printing apparatus which have one or more of the following technical features that are described in below in respective paragraphs given parenthesized sequential numbers (1) to (19). Any technical feature that includes another technical feature shall do so by referring, at the beginning, to the parenthesized sequential number given to the latter feature. Thus, two or more of the following technical features may be combined, if appropriate. Each technical feature may be accompanied by a supplemental explanation, as needed. However, the following technical features and the appropriate combinations thereof are just examples to which the present invention is by no means limited. In addition, in the case where one technical feature recites a plurality of items, it is not essentially required that all of those items be simultaneously employed. That is, it is possible to select and employ only a portion (one, two, but not all) of those items.
(1) According to a first feature of the present invention, there is provided a method of printing a printing material on an object, comprising the steps of printing the printing material on the object through a mask having at least one through-hole formed through a thickness thereof; inspecting the printing material printed on at least one portion of the object through the at least one through-hole of the mask; and re-printing, when the inspection shows that an amount of the printing material printed on the at least one portion of the object is short, the printing material on the at least one portion of the object through the at least one through-hole of the mask to compensate for the short amount of the printing material.
The printing material may be a creamed solder or an adhesive. The object may be a printed wiring board (xe2x80x9cPWBxe2x80x9d), or an externally exposed substrate of a xe2x80x9cleadlessxe2x80x9d electric component having no lead wires. The leadless electric component is a package electric component which is provided by a container and chip components held by the container. The mask may be a screen mask formed of fibers, a combination mask obtained by reinforcing a screen mask with metal, or a stencil mask formed of metal. The shortage of the amount of the printing material may be compensated, according to the second feature (2) described below, by carrying out the mask-printing operation once more over the entire portion of the object. In an alternative manner, the mask is not used, and a device which can print the printing material on only a dandsired portion of the object is employed, so that the printing material is printed on only one or more print spots where the shortage of printing-material amount has been found. The inspection may be carried out on all the print spots of the object where the printing material has been printed, or on only a portion (one, two, . . ., but not all) of the print spots. In the latter case, the inspection may be carried out on a particular portion of the print spots where the shortage of printing-material amount is likely to occur, or a portion of the print spots that is random-sampled for each of objects. The re-printing can compensate for the short amount of printing material, and the object on which the shortage of printing-material amount has occurred need not be discarded as a defective. Thus, in the case where the object is a PWB and the printing of the printing material is carried out as one step of a printed-circuit board producing method, the number of PWBs that are discarded from the production line can be largely reduced.
(2) According to a second feature of the present invention that includes the first feature (1), the printing step comprises printing the printing material on a plurality of portions of the objects through the mask having a plurality of through-holes formed through the thickness thereof, the inspecting step comprises inspecting the printing material printed on at least one of the plurality of portions of the object through at least a corresponding one of the plurality of through-holes of the mask, and the re-printing step comprises re-printing, when the inspection shows that an amount of the printing material printed on the at least one portion of the object is short, the printing material on each of the plurality of portions of the object through a corresponding one of the plurality of through-holes of the mask.
The re-printing step may be the same step as the printing step and accordingly may be easily carried out by repeating the printing step.
(3) According to a third feature of the present invention that includes the second feature (2), the re-printing step comprises re-printing the printing material on the each portion of the object through the corresponding one through-hole of the mask, such that an amount of the printing material squeezed into the one through-hole in the re-printing step is smaller than the amount of the printing material squeezed into the one through-hole in the printing step.
This feature may be achieved by, e.g., decreasing the pressure under which the printing material is squeezed into the through-hole, or the time duration during which the printing material is squeezed into the through-hole. Since some amount of the printing material is, though being short, already present on the object, a total amount of the printing material printed on the object would be excessive if the same amount of printing material as that printed in the printing step is printed in the re-printing step. However, this feature is free of the problem that an excessive amount of printing material is printed on an object.
(4) According to a fourth feature of the present invention that includes any one of the first to third features (1) to (3), the inspecting step comprises taking, with an image taking device, an image of a surface of the object on which the printing material has been printed, and finding, based on the taken image, the shortage of the amount of the printing material.
The shortage of the printing material printed on the object can occur in various manners. For example, the height of the printing material is insufficient, or the printing material has a chip, i.e., no printing material is printed on at least a portion of a print area on which the material should be printed. The image taking device may be provided by a two-dimensional-image taking device which is used in combination with a planar-light source; or a laser displacement sensor disclosed in the above-identified Japanese Patent document No. 5-50578. The laser displacement sensor includes a laser-beam generator, an optical system, and a reflected-light converging device, emits a laser beam toward the printing material, and takes an image of the printing material based on the light or laser beam reflected from the material. To take an image of the material, at least one of the sensor and the material is moved relative to the other of the sensor and the material. Otherwise, the image taking device may be one which takes a plan image of the printing material. The last image taking device can find the chip of the printing material. In the last case, at least one of the image taking device and the printing material printed on the object is moved relative to the other of the device and the material.
(5) According to a fifth feature of the present invention that includes the fourth features (4), the inspecting step comprises separating, without moving the object and the mask relative to each other in a direction parallel to the object and the mask, at least one of the object and the mask from the other of the object and the mask in a direction perpendicular to the object and the mask, and moving the image taking device into a space produced between the object and the mask.
According to this feature, the inspection can be quickly performed without changing the relative position between the object and the mask in any directions parallel to the object and the mask.
(6) According to a sixth feature of the present invention that includes the fourth features (4), the inspecting step comprises moving at least one of the object and the mask relative to the other of the object and the mask, in a direction parallel to the object and the mask, and taking, with the image taking device, the image of the surface of the object in a state in which the mask is not present above the object.
According to this feature, the inspection can be performed in the state in which the mask is not present above the object. Thus, the inspecting step can employ an inspecting device having a simple construction, or can enjoy a high degree of freedom with which an inspecting device is disposed.
(7) According to a seventh feature of the present invention that includes the sixth feature (6), the printing step comprises printing the printing material on a plurality of portions of the objects through the mask having a plurality of through-holes formed through the thickness thereof, the inspecting step comprises inspecting the printing material printed on at least one of the plurality of portions of the object through at least a corresponding one of the plurality of through-holes of the mask, the re-printing step comprises re-printing, when the inspection shows that an amount of the printing material printed on the at least one portion of the object is short, the printing material on each of the plurality of portions of the object through a corresponding one of the plurality of through-holes of the mask, and the method further comprises a step of positioning, before starting the printing step, at least one of the object and the mask relative to the other of the object and the mask, and a step of repositioning, before starting the re-printing step, at least one of the object and the mask relative to the other of the object and the mask.
Before the printing step is started, at least one of the object and the mask is positioned relative to the other of the object and the mask in a direction parallel to the object and the mask. However, to perform the inspection, at least one of the object and the mask is moved relative to the other of the object and the mask, in the direction parallel to the object and the mask, and accordingly the object and the mask may be out of position relative to each other. Therefore, before the re-printing step is started, at least one of the object and the mask is re-positioned relative to the other of the object and the mask, so that the printing material can be accurately printed on a print spot on the object.
(8) According to an eighth feature of the present invention that includes any one of the first to seventh features (1) to (7), the printing method further comprises a step of cleaning the mask, at least when the inspection shows the shortage of the printing amount.
It is speculated that one of the causes of the shortage of the printing material is the sticking of the printing material to the inner surface of the through-hole of the mask. The shortage of the printing material that is caused by the sticking of the material can be removed by cleaning the mask and thereby removing the material stuck to the mask. However, it is possible to clean the mask at a time or times different from the time when the shortage of the printing material is found.
(9) According to a ninth feature of the present invention that includes any one of the first to eighth features (1) to (8), the inspecting step comprises inspecting the printing material for finding at least one different sort of defect than the shortage of the amount of the printing material, and identifying a particular sort of the defect found.
Since the particular sort of the found defect is identified, an operator can take appropriate countermeasures against the identified sort of defect.
(10) According to a tenth feature of the present invention that includes the ninth feature (9), the at least one different sort of defect comprises at least one of an excessive amount of the printing material printed on the object, and a printing of the printing material at an erroneous position on the object.
The mask-printing method according to any one of the first to tenth features can employ any one of the eleventh to nineteenth features that will be described below in connection with the mask-printing apparatus, and the mask-printing apparatus according to any one of the eleventh to nineteenth features can employ any one of the first to tenth features that have been described above in association with the mask-printing method.
(11) According to an eleventh feature of the present invention, there is provided a mask printing apparatus comprising an object supporting device which supports an object; a printing device which prints, through a mask having at least one through-hole formed through a thickness thereof, a printing material on the object supported by the object supporting device; an inspecting device which inspects the printing material printed on at least one portion of the object through the at least one through-hole of the mask; and a re-printing device which re-prints, when the inspecting device finds that an amount of the printing material printed on the at least one portion of the object is short, the printing material on the at least one portion of the object through the at least one through-hole of the mask to compensate for the short amount of the printing material.
The mask-printing apparatus according to the eleventh feature (11) can advantageously perform the mask-printing method according to the first feature (1).
(12) According to a twelfth feature of the present invention that includes the eleventh feature (11), the inspecting device comprises an image taking device which takes an image of the printing material printed on the object by the printing device, and a data processor which processes image data representing the image taken by the image taking device to find the shortage of the amount of the printing material.
The image taking device may be one of the various devices described above in connection with the fourth feature (4).
(13) According to a thirteenth feature of the present invention that includes the eleventh or twelfth feature (11) or (12), the printing apparatus further comprises a positioning device which automatically moves at least one of the object supporting device and the mask relative to the other of the object supporting device and the mask, in a direction parallel to the object supported by the object supporting device, and the mask, and thereby positions at least one of the object and the mask relative to the other of the object and the mask.
As will be described in DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS, at least one of the object and the mask may be positioned relative to the other of the object and the mask, before the re-printing, irrespective of whether the inspection is performed at a position different from the position where the mask printing is performed, or the very position where the mask printing is performed. Since at least one of the object supporting device and the mask is positioned relative to the other of the object supporting device and the mask, the printing material can be accurately printed on a print spot on the object.
(14) According to a fourteenth feature of the present invention that includes the thirteenth feature (13), the positioning device comprises at least one image taking device which takes a first image of at least one reference mark provided on the object supported by the object supporting device, and a second image of at least one reference mark provided on the mask; and a positional-error finding device which finds, based on the first and second images taken by the image taking device, a positional error of the object and the mask relative to each other.
The image taking device which takes the first and second images may be provided by either the above-described image taking device which takes the image of the printing material, or a device different from the above-described image taking device. In addition, the at least one image taking device which takes the first and second images may be provided by respective exclusive image taking devices which takes the firs and second images, respectively; or a single, common image taking device which takes both the first and second images. In the latter case, the common image taking device may be selectively oriented in each of two opposite directions, or may be selectively used with each of two optical systems.
(15) According to a fifteenth feature of the present invention that includes any one of the eleventh to fourteenth features (11) to (14), the printing device prints the printing material on a plurality of portions of the objects through the mask having a plurality of through-holes formed through the thickness thereof, the inspecting device inspects the printing material printed on at least one of the plurality of portions of the object through at least a corresponding one of the plurality of through-holes of the mask, the re-printing device comprises the printing device and a re-printing commanding device which commands, when the inspecting device finds that an amount of the printing material printed on the at least one portion of the object is short, the printing device to reprint the printing material on each of the plurality of portions of the object through a corresponding one of the plurality of through-holes of the mask.
The mask-printing apparatus according to the fifteenth feature (15) can advantageously perform the mask-printing method according to the second feature (2).
(16) According to a sixteenth feature of the present invention that includes the fifteenth feature (15), the re-printing commanding device comprises a smaller-amount-printing commanding device which commands the printing device to re-print the printing material on the each portion of the object through the corresponding one through-hole of the mask, such that an amount of the printing material squeezed into the one through-hole for the re-printing is smaller than the amount of the printing material squeezed into the one through-hole for the printing.
The mask-printing apparatus according to the sixteenth feature (16) can advantageously perform the mask-printing method according to the third feature (3).
(17) According to a seventeenth feature of the present invention that includes any one of the eleventh to sixteenth features (11) to (16), the printing apparatus further comprises a cleaning device which automatically cleans the mask.
The cleaning device may be any one of various devices. As will be described in DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS, the cleaning device may be one which cleans the mask remaining at a first position where the mask is used for the printing, without having to move the object and the mask relative to each other in a direction parallel to the object and the mask; or one which is provided at a second position away from the first position where the mask is used for the printing and which cleans the mask at the second position. In the latter case, the mask printing may be suspended while the current mask is cleaned by the cleaning device; or the mask printing may be performed by a spare mask, while the current mask is cleaned. The mask-printing apparatus according to the seventeenth feature (17) can advantageously perform the mask-printing method according to the eighth feature (8).
(18) According to an eighteenth feature of the present invention that includes the seventeenth feature (17), the printing apparatus further comprises a cleaning commanding device which commands, at least when the inspecting device finds the shortage of the amount of the printing material, the cleaning device to clean the mask.
(19) According to a nineteenth feature of the present invention that includes any one of the eleventh to eighteenth features (11) to (18), the inspecting device is provided at a position distant from the printing device in a direction parallel to the mask, and comprises a moving device which moves the object from the printing device to the inspecting device, and the printing apparatus further comprises an object returning device which returns, when the inspecting device finds the shortage of the amount of the printing material, the object to the printing device.
As will be described in DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS, the object returning device may be provided by a conveying device which carries, in and carries out, the object to a position where the printing and the re-printing are performed; or a device different from the conveying device. The mask-printing apparatus according to the nineteenth feature (19) can advantageously perform the mask-printing method according to the sixth feature (6). In the case where the printing apparatus further comprises the above-described positioning device, the re-printing can be accurately performed.